Allow users to add content to an existing node?

Hello,

In order for my website to function, users need to be able to add content to an existing node - not edit it. In addition to this, the author of the original node must be able to select/confirm the added content.

A quick example of this would be similar to how Freelancer.com works. A user posts an advert requiring work carrying out, potential workers then have the option to leave there details and experience for the original advert author to revise through and ultimately, accept/reject applications.

Is this achievable with Drupal? How would I approach it?

Many thanks for your time.